There is growing interest in materials which have high electro-optical (EO) and nonlinear optical (NLO) responses. These optically active materials have conventionally been of inorganic nature, such as LiNbO.sub.3, in which the EO response, caused by atomic motions in the crystalline lattice, is slow compared to optical frequencies. Of more recent development are organic polymeric EO materials. In these, the rapid moving .pi.-electrons in a conjugated framework are responsible for the EO properties. The advantages of using EO polymeric materials lie in their superior EO properties (large EO response over a wide frequency range, fast response time, low dielectric constant, higher laser damage threshold, etc.), better processing characteristics, compatibility with a variety of substrates, and the opportunity to manipulate organic molecular structures to optimize the above properties.
Electro-optically responsive polymeric media have been developed in which a particular molecular entity or functionality, denoted here by the term "molecular electro-optical transducer", is chemically incorporated into an amorphous polymer. The polymer may be fabricated into a suitable form (typically a thin film formed by spin-casting). If such material is "poled", that is to say it is heated to a temperature near or above its glass transition temperature, T.sub.g, and is subjected to an applied electric field of appropriate magnitude for a suitable period of time, and is thereafter cooled to temperatures considerably below the T.sub.g, a non-centrosymetric orientation is imparted to the molecular transducer and the material acquires an electro-optic response, that is, the index of refraction of the material will change in direct linear response to an applied electric field. According to a theory [K. D. Singer et al., Appl. Phys. Lett. 49, (1986), 248] which has been said to apply to this situation, the major electro-optic coefficient is determined by the following factors:
(1) number density of molecular transducers, N/V (molecules/cm.sup.3); PA0 (2) molecular hyperpolarizability of the transducer, .beta..sub.EO (esu); PA0 (3) permanent dipole moment of the transducer, .mu. (esu or debye); PA0 (4) applied electric field during "poling", E.sub.p (statvolt/cm); PA0 (5) glass transition temperature, T.sub.g (.degree.K.); PA0 (6) indices of refraction of the medium, n; PA0 (7) "local field factors, f, which for polymer systems are generally approximately unity. PA0 (b) Ar is selected from the group consisting of ##STR2## wherein Z.sup.2 is (i) --H PA0 (e) R.sup.2 is selected from the group consisting of PA0 (a) monomeric materials tend to evaporate or "bake out" during the baking processes normally employed to remove solvent; PA0 (b) monomeric materials tend to aggregate or phase separate--especially at concentrations about 10% by weight; PA0 (c) solid solutions at high concentrations of solute generally have poorer physical properties and poorer physical integrity; PA0 (d) solid solutions potentially have greater health and safety hazard to those working with them as some of these monomeric materials might be expected to be carcinogens; PA0 (e) poled films of solid solutions are reported to "relax" to less ordered states more rapidly than polymeric versions.
According to this theory, the observed electro-optic coefficient should be given by: EQU r.apprxeq.(N/V)(.mu.E.sub.p /5kT.sub.g).beta..sub.EO
Thus r is optimized by maximizing (N/V), .mu., E.sub.p, and importantly .beta..sub.EO and by minimizing T.sub.g (subject to restriction that T.sub.g &gt;temperatures of use).
The exact nature of the molecular characteristics which determine .beta..sub.EO has not been fully elucidated. However, on the basis of a simple theory it is said that transducers with large .beta. are characterized by a large permanent dipole change upon electronic excitation, by a strong oscillator strength for the lowest energy electronic absorption, and by relative closeness of the lowest energy absorption wavelength to the incident wavelength (limited however by the loss of incident radiation by absorption if the absorption wavelength is too close to the incident wavelength).
For electro-optic processes, the molecular transducer must have a dipole with delocalized electron density. Typically, the dipole is constructed by attaching electron-donating (EDG) and electron-withdrawing (EWG) substituents at either end of a conjugated (.pi.) framework. The NLO activity of the transducer as measured from .beta. is determined by the choice of the EDG and EWG, and the length of the .pi.-system. Higher activity is achieved with stronger donor and acceptor and longer .pi.-chain. Typical electron-donating substituents are --NH.sub.2, NHR, --NR.sub.2, --SR, --OR, etc. Typical electron-withdrawing substituents include --CN, --NO.sub.2, --CO.sub.2 R, --C(CN).dbd.C(CN).sub.2, etc. Typical .pi.-units are alkynyl (--C.dbd.C--), alkenyl and heteroalkenyl (such as --CH.dbd.CH--, --N.dbd.N--, --CH.dbd.N--); aryl and heteroaryl (such as phenyl, biphenyl, pyridinyl) and their various combinations (such as aryl-CH.dbd.CH-aryl, aryl-N.dbd.N-aryl, aryl-CH.dbd.N-aryl).
The prior art describes a number of molecular transducing groups useful for creating electro-optic polymers. The prior art also teaches that the transducer may be attached through a chemical "handle" directly to a pre-existing polymer or to a suitable polymer precursor (monomer) which could later be converted into a polymer. The transducers of the prior art include structures of the type EWG-phenyl-azo-phenyl-NR.sub.2 wherein EWG denotes an electron withdrawing group. We have found that by making certain modifications to the azo-based transducers, new transducers having enhanced electro-optical properties are obtained. These new transducers, compositions and polymers based thereon, monomeric compositions for making such polymers, and utilization thereof in light modulator devices, especially electro-optical light modulator devices, are the objects of the present invention, as to be described in detail below.